In a GSM/GPRS/EGPRS communication system, it is well known that the signal received by a mobile station from a base station, or by a base station from a mobile station, is corrupted by various distortions in its propagation. Amongst these distortions, one of the most severe is multipath propagation, whereby the received signal is corrupted by multiple reflections from large objects, such as buildings, in the environment, which reflections are received with different delays. The range of delays encountered is significant compared with the duration of a single digital symbol being transmitted, and means have to be provided to compensate for, or “equalize”, this distortion. Commonly, this is achieved by inserting a known “training sequence” at the centre of each transmitted burst. As this sequence is a known signal, the propagation path can be characterized, and then compensated for, by known equalization techniques.
A further distortion introduced into the received signal is DC offset. This can arise for several reasons. For example, most receivers today use direct conversion wherein a received bandpass RF signal is converted into a quadrature representation folded around zero frequency by multiplying by a quadrature local oscillator signal nominally at the channel frequency. The analogue multipliers used are seldom perfect and exhibit DC errors, which may furthermore vary with signal level and temperature. Also, any local oscillator signals that leak into the RF signal path become converted to DC offsets. Also, DC offsets may arise from non-linear distortion of the received signal.
DC offsets have a severe effect on the signal demodulator in a GSM/GPRS/EGPRS system and have to be removed. This can be achieved, to a degree, by analogue signal processing prior to analogue-to-digital conversion.
EGPRS (enhanced general packet radio service) is an evolution towards higher rate transmission for GSM/GPRS mobile communication systems. It introduces several enhancements to increase the spectrum efficiency using the same spectrum otherwise used for conventional GSM transmission, one of which is the introduction of the 3π/8 offset 8PSK modulation scheme in addition to the GMSK modulation used in GSM/GPRS. In ideal conditions (no noise or other transmission artifacts) 8PSK can offer up to three times higher data rate than GSMK modulation. However, 3π/8 offset 8PSK is more sensitive to noise, frequency offset and added DC offset than conventional GMSK.